1. Field of the Invention
The present invention generally relates to a digital magnetic recording and/or reproducing apparatus for use with, for example, a digital video tape recorder (digital VTR) and, more particularly, is directed to a rotary magnetic head drum for a recording and/or reproducing apparatus.
2. Description of the Prior Art
Recently, so-called digital video tape recorders (digital VTRs) have been developed to record and/or reproduce, for example, a video signal on and/or from a magnetic tape in the form of a digital video signal by utilizing rotary heads. Such conventional digital VTR will be described with reference to FIG. 1.
As shown in FIG. 1, in the conventional digital VTR, two heads 21a and 21b are mounted on a rotary drum 22 at an angle of 180 degrees from each other, and a magnetic tape 23 is wrapped around the rotary drum 22 with a wrapping angle of 180 degrees and is transported at a predetermined tape speed so that, as shown in FIG. 2, recording tracks 24a and 24b are formed on the magnetic tape 23 by the heads 21a and 21b.
In most conventional VTRs, a so-called ball bearing is employed as a bearing of the rotary drum 22. But in digital VTRs, the diameter of rotary drum 22 is reduced, the rotary drum 22 is rotated at high speed and the track pitch is reduced, all of which requires a bearing which can be rotated silently and with high accuracy. A conventional ball bearing, however, cannot meet the above requirements because the balls thereof produce noise and cannot be satisfactorily reduced in size.
The above shortcoming and disadvantages can be overcome by the use of a dynamic pressure bearing as the bearing of the digital VTR. In a dynamic pressure bearing, a shaft rotatably supports a sleeve or bushing with a layer of a fluid (lubricant) therebetween. Thus, the size of the bearing can be determined with relatively large freedom in accordance with the diameter of the rotary drum. Further, since the shaft and sleeve rotate relative to each other without metal-to-metal contact, it is possible to attain high speed rotation without noise. Moreover, the characteristics of the dynamic pressure bearing can be varied by changing the lubricant and thus, the dynamic pressure bearing has various advantages.
However, when the above dynamic pressure bearing is used as the bearing of a rotary drum assembly which, for example, is composed of an upper rotating drum and a lower stationary drum in a digital VTR, in the stop mode (stationary state), the dynamic-pressure bearing permits the upper rotating drum to incline relative to the shaft. If a dynamic pressure bearing having the above-mentioned feature is applied to the shaft of the rotary drum 22 with the diametrically opposed magnetic heads 21a and 21b shown in FIG. 1 or to a head system in which pairs of heads are opposed to each other by 180 degrees, the pairing adjustment made in the stationary state is displaced relative to the head positions during operation by an amount corresponding to the inclined amount of the upper drum relative to its position in the operation mode. Although the inclined amount is determined on the basis of bearing gap, bearing length and drum diameter, the amount of displacement in the height direction falls in a range of from several micrometers to several tens of micrometers, thus considerably degrading the accuracy of the bearing. Accordingly, the track pitch cannot be reduced and the recording density cannot be increased, which are indispensable conditions for a digital VTR.